DESCRIPTION

Like many languages, Perl provides for user-defined subroutines. These may be located anywhere in the main program, loaded in from other files via the do, require, or use keywords, or even generated on the fly using eval or anonymous subroutines (closures). You can even call a function indirectly using a variable containing its name or a CODE reference to it, as in $var = \&function.

The Perl model for function call and return values is simple: all functions are passed as parameters one single flat list of scalars, and all functions likewise return to their caller one single flat list of scalars. Any arrays or hashes in these call and return lists will collapse, losing their identities--but you may always use pass-by-reference instead to avoid this. Both call and return lists may contain as many or as few scalar elements as you'd like. (Often a function without an explicit return statement is called a subroutine, but there's really no difference from the language's perspective.)

Any arguments passed to the routine come in as the array @_. Thus if you called a function with two arguments, those would be stored in $_[0] and $_[1]. The array @_ is a local array, but its elements are aliases for the actual scalar parameters. In particular, if an element $_[0] is updated, the corresponding argument is updated (or an error occurs if it is not updatable). If an argument is an array or hash element which did not exist when the function was called, that element is created only when (and if) it is modified or if a reference to it is taken. (Some earlier versions of Perl created the element whether or not it was assigned to.) Note that assigning to the whole array @_ removes the aliasing, and does not update any arguments.

The return value of the subroutine is the value of the last expression evaluated. Alternatively, a return statement may be used to exit the subroutine, optionally specifying the returned value, which will be evaluated in the appropriate context (list, scalar, or void) depending on the context of the subroutine call. If you specify no return value, the subroutine will return an empty list in a list context, an undefined value in a scalar context, or nothing in a void context. If you return one or more arrays and/or hashes, these will be flattened together into one large indistinguishable list.

Perl does not have named formal parameters, but in practice all you do is assign to a my() list of these. Any variables you use in the function that aren't declared private are global variables. For the gory details on creating private variables, see "Private Variables via my()" and "Temporary Values via local()". To create protected environments for a set of functions in a separate package (and probably a separate file), see "Packages" in perlmod.

This also has the effect of turning call-by-reference into call-by-value, because the assignment copies the values. Otherwise a function is free to do in-place modifications of @_ and change its caller's values.

Notice how this (unprototyped) function doesn't care whether it was passed real scalars or arrays. Perl will see everything as one big long flat @_ parameter list. This is one of the ways where Perl's simple argument-passing style shines. The upcase() function would work perfectly well without changing the upcase() definition even if we fed it things like this:

Because like its flat incoming parameter list, the return list is also flat. So all you have managed to do here is stored everything in @a and made @b an empty list. See "Pass by Reference" for alternatives.

A subroutine may be called using the "&" prefix. The "&" is optional in modern Perls, and so are the parentheses if the subroutine has been predeclared. (Note, however, that the "&" is NOT optional when you're just naming the subroutine, such as when it's used as an argument to defined() or undef(). Nor is it optional when you want to do an indirect subroutine call with a subroutine name or reference using the &$subref() or &{$subref}() constructs. See perlref for more on that.)

Subroutines may be called recursively. If a subroutine is called using the "&" form, the argument list is optional, and if omitted, no @_ array is set up for the subroutine: the @_ array at the time of the call is visible to subroutine instead. This is an efficiency mechanism that new users may wish to avoid.

Not only does the "&" form make the argument list optional, but it also disables any prototype checking on the arguments you do provide. This is partly for historical reasons, and partly for having a convenient way to cheat if you know what you're doing. See the section on Prototypes below.

A "my" declares the listed variables to be confined (lexically) to the enclosing block, conditional (if/unless/elsif/else), loop (for/foreach/while/until/continue), subroutine, eval, or do/require/use'd file. If more than one value is listed, the list must be placed in parentheses. All listed elements must be legal lvalues. Only alphanumeric identifiers may be lexically scoped--magical builtins like $/ must currently be localized with "local" instead.

Unlike dynamic variables created by the "local" statement, lexical variables declared with "my" are totally hidden from the outside world, including any called subroutines (even if it's the same subroutine called from itself or elsewhere--every call gets its own copy).

(An eval(), however, can see the lexical variables of the scope it is being evaluated in so long as the names aren't hidden by declarations within the eval() itself. See perlref.)

The parameter list to my() may be assigned to if desired, which allows you to initialize your variables. (If no initializer is given for a particular variable, it is created with the undefined value.) Commonly this is used to name the parameters to a subroutine. Examples:

The "my" is simply a modifier on something you might assign to. So when you do assign to the variables in its argument list, the "my" doesn't change whether those variables is viewed as a scalar or an array. So

my ($foo) = <STDIN>;
my @FOO = <STDIN>;

both supply a list context to the right-hand side, while

my $foo = <STDIN>;

supplies a scalar context. But the following declares only one variable:

my $foo, $bar = 1;

That has the same effect as

my $foo;
$bar = 1;

The declared variable is not introduced (is not visible) until after the current statement. Thus,

my $x = $x;

can be used to initialize the new $x with the value of the old $x, and the expression

my $x = 123 and $x == 123

is false unless the old $x happened to have the value 123.

Lexical scopes of control structures are not bounded precisely by the braces that delimit their controlled blocks; control expressions are part of the scope, too. Thus in the loop

the scope of $answer extends from its declaration throughout the rest of the conditional (including elsif and else clauses, if any), but not beyond it.

(None of the foregoing applies to if/unless or while/until modifiers appended to simple statements. Such modifiers are not control structures and have no effect on scoping.)

The foreach loop defaults to scoping its index variable dynamically (in the manner of local; see below). However, if the index variable is prefixed with the keyword "my", then it is lexically scoped instead. Thus in the loop

for my $i (1, 2, 3) {
some_function();
}

the scope of $i extends to the end of the loop, but not beyond it, and so the value of $i is unavailable in some_function().

Some users may wish to encourage the use of lexically scoped variables. As an aid to catching implicit references to package variables, if you say

use strict 'vars';

then any variable reference from there to the end of the enclosing block must either refer to a lexical variable, or must be fully qualified with the package name. A compilation error results otherwise. An inner block may countermand this with "no strict 'vars'".

A my() has both a compile-time and a run-time effect. At compile time, the compiler takes notice of it; the principle usefulness of this is to quiet use strict 'vars'. The actual initialization is delayed until run time, so it gets executed appropriately; every time through a loop, for example.

Variables declared with "my" are not part of any package and are therefore never fully qualified with the package name. In particular, you're not allowed to try to make a package variable (or other global) lexical:

In fact, a dynamic variable (also known as package or global variables) are still accessible using the fully qualified :: notation even while a lexical of the same name is also visible:

package main;
local $x = 10;
my $x = 20;
print "$x and $::x\n";

That will print out 20 and 10.

You may declare "my" variables at the outermost scope of a file to hide any such identifiers totally from the outside world. This is similar to C's static variables at the file level. To do this with a subroutine requires the use of a closure (anonymous function). If a block (such as an eval(), function, or package) wants to create a private subroutine that cannot be called from outside that block, it can declare a lexical variable containing an anonymous sub reference:

As long as the reference is never returned by any function within the module, no outside module can see the subroutine, because its name is not in any package's symbol table. Remember that it's not REALLY called $some_pack::secret_version or anything; it's just $secret_version, unqualified and unqualifiable.

This does not work with object methods, however; all object methods have to be in the symbol table of some package to be found.

Just because the lexical variable is lexically (also called statically) scoped doesn't mean that within a function it works like a C static. It normally works more like a C auto. But here's a mechanism for giving a function private variables with both lexical scoping and a static lifetime. If you do want to create something like C's static variables, just enclose the whole function in an extra block, and put the static variable outside the function but in the block.

If this function is being sourced in from a separate file via require or use, then this is probably just fine. If it's all in the main program, you'll need to arrange for the my() to be executed early, either by putting the whole block above your main program, or more likely, placing merely a BEGIN sub around it to make sure it gets executed before your program starts to run:

Temporary Values via local()

NOTE: In general, you should be using "my" instead of "local", because it's faster and safer. Exceptions to this include the global punctuation variables, filehandles and formats, and direct manipulation of the Perl symbol table itself. Format variables often use "local" though, as do other variables whose current value must be visible to called subroutines.

A local() modifies its listed variables to be local to the enclosing block, (or subroutine, eval{}, or do) and any called from within that block. A local() just gives temporary values to global (meaning package) variables. This is known as dynamic scoping. Lexical scoping is done with "my", which works more like C's auto declarations.

If more than one variable is given to local(), they must be placed in parentheses. All listed elements must be legal lvalues. This operator works by saving the current values of those variables in its argument list on a hidden stack and restoring them upon exiting the block, subroutine, or eval. This means that called subroutines can also reference the local variable, but not the global one. The argument list may be assigned to if desired, which allows you to initialize your local variables. (If no initializer is given for a particular variable, it is created with an undefined value.) Commonly this is used to name the parameters to a subroutine. Examples:

Because local() is a run-time command, it gets executed every time through a loop. In releases of Perl previous to 5.0, this used more stack storage each time until the loop was exited. Perl now reclaims the space each time through, but it's still more efficient to declare your variables outside the loop.

A local is simply a modifier on an lvalue expression. When you assign to a localized variable, the local doesn't change whether its list is viewed as a scalar or an array. So

local($foo) = <STDIN>;
local @FOO = <STDIN>;

both supply a list context to the right-hand side, while

local $foo = <STDIN>;

supplies a scalar context.

A note about local() and composite types is in order. Something like local(%foo) works by temporarily placing a brand new hash in the symbol table. The old hash is left alone, but is hidden "behind" the new one.

This means the old variable is completely invisible via the symbol table (i.e. the hash entry in the *foo typeglob) for the duration of the dynamic scope within which the local() was seen. This has the effect of allowing one to temporarily occlude any magic on composite types. For instance, this will briefly alter a tied hash to some other implementation:

Passing Symbol Table Entries (typeglobs)

[Note: The mechanism described in this section was originally the only way to simulate pass-by-reference in older versions of Perl. While it still works fine in modern versions, the new reference mechanism is generally easier to work with. See below.]

Sometimes you don't want to pass the value of an array to a subroutine but rather the name of it, so that the subroutine can modify the global copy of it rather than working with a local copy. In perl you can refer to all objects of a particular name by prefixing the name with a star: *foo. This is often known as a "typeglob", because the star on the front can be thought of as a wildcard match for all the funny prefix characters on variables and subroutines and such.

When evaluated, the typeglob produces a scalar value that represents all the objects of that name, including any filehandle, format, or subroutine. When assigned to, it causes the name mentioned to refer to whatever "*" value was assigned to it. Example:

Note that scalars are already passed by reference, so you can modify scalar arguments without using this mechanism by referring explicitly to $_[0] etc. You can modify all the elements of an array by passing all the elements as scalars, but you have to use the * mechanism (or the equivalent reference mechanism) to push, pop, or change the size of an array. It will certainly be faster to pass the typeglob (or reference).

Even if you don't want to modify an array, this mechanism is useful for passing multiple arrays in a single LIST, because normally the LIST mechanism will merge all the array values so that you can't extract out the individual arrays. For more on typeglobs, see "Typeglobs and Filehandles" in perldata.

Pass by Reference

If you want to pass more than one array or hash into a function--or return them from it--and have them maintain their integrity, then you're going to have to use an explicit pass-by-reference. Before you do that, you need to understand references as detailed in perlref. This section may not make much sense to you otherwise.

Here are a few simple examples. First, let's pass in several arrays to a function and have it pop all of then, return a new list of all their former last elements:

So far, we're using just the normal list return mechanism. What happens if you want to pass or return a hash? Well, if you're using only one of them, or you don't mind them concatenating, then the normal calling convention is ok, although a little expensive.

Where people get into trouble is here:

(@a, @b) = func(@c, @d);
or
(%a, %b) = func(%c, %d);

That syntax simply won't work. It sets just @a or %a and clears the @b or %b. Plus the function didn't get passed into two separate arrays or hashes: it got one long list in @_, as always.

If you can arrange for everyone to deal with this through references, it's cleaner code, although not so nice to look at. Here's a function that takes two array references as arguments, returning the two array elements in order of how many elements they have in them:

Here we're using the typeglobs to do symbol table aliasing. It's a tad subtle, though, and also won't work if you're using my() variables, because only globals (well, and local()s) are in the symbol table.

If you're passing around filehandles, you could usually just use the bare typeglob, like *STDOUT, but typeglobs references would be better because they'll still work properly under use strict 'refs'. For example:

Although that will actually produce a small memory leak. See the bottom of "open()" in perlfunc for a somewhat cleaner way using the IO::Handle package.

Prototypes

As of the 5.002 release of perl, if you declare

sub mypush (\@@)

then mypush() takes arguments exactly like push() does. The declaration of the function to be called must be visible at compile time. The prototype affects only the interpretation of new-style calls to the function, where new-style is defined as not using the & character. In other words, if you call it like a builtin function, then it behaves like a builtin function. If you call it like an old-fashioned subroutine, then it behaves like an old-fashioned subroutine. It naturally falls out from this rule that prototypes have no influence on subroutine references like \&foo or on indirect subroutine calls like &{$subref}.

Method calls are not influenced by prototypes either, because the function to be called is indeterminate at compile time, because it depends on inheritance.

Because the intent is primarily to let you define subroutines that work like builtin commands, here are the prototypes for some other functions that parse almost exactly like the corresponding builtins.

Any backslashed prototype character represents an actual argument that absolutely must start with that character. The value passed to the subroutine (as part of @_) will be a reference to the actual argument given in the subroutine call, obtained by applying \ to that argument.

Unbackslashed prototype characters have special meanings. Any unbackslashed @ or % eats all the rest of the arguments, and forces list context. An argument represented by $ forces scalar context. An & requires an anonymous subroutine, which, if passed as the first argument, does not require the "sub" keyword or a subsequent comma. A * does whatever it has to do to turn the argument into a reference to a symbol table entry.

A semicolon separates mandatory arguments from optional arguments. (It is redundant before @ or %.)

Note how the last three examples above are treated specially by the parser. mygrep() is parsed as a true list operator, myrand() is parsed as a true unary operator with unary precedence the same as rand(), and mytime() is truly without arguments, just like time(). That is, if you say

mytime +2;

you'll get mytime() + 2, not mytime(2), which is how it would be parsed without the prototype.

The interesting thing about & is that you can generate new syntax with it:

That prints "unphooey". (Yes, there are still unresolved issues having to do with the visibility of @_. I'm ignoring that question for the moment. (But note that if we make @_ lexically scoped, those anonymous subroutines can act like closures... (Gee, is this sounding a little Lispish? (Never mind.))))

Some folks would prefer full alphanumeric prototypes. Alphanumerics have been intentionally left out of prototypes for the express purpose of someday in the future adding named, formal parameters. The current mechanism's main goal is to let module writers provide better diagnostics for module users. Larry feels the notation quite understandable to Perl programmers, and that it will not intrude greatly upon the meat of the module, nor make it harder to read. The line noise is visually encapsulated into a small pill that's easy to swallow.

It's probably best to prototype new functions, not retrofit prototyping into older ones. That's because you must be especially careful about silent impositions of differing list versus scalar contexts. For example, if you decide that a function should take just one parameter, like this:

sub func ($) {
my $n = shift;
print "you gave me $n\n";
}

and someone has been calling it with an array or expression returning a list:

func(@foo);
func( split /:/ );

Then you've just supplied an automatic scalar() in front of their argument, which can be more than a bit surprising. The old @foo which used to hold one thing doesn't get passed in. Instead, the func() now gets passed in 1, that is, the number of elements in @foo. And the split() gets called in a scalar context and starts scribbling on your @_ parameter list.

This is all very powerful, of course, and should be used only in moderation to make the world a better place.

Constant Functions

Functions with a prototype of () are potential candidates for inlining. If the result after optimization and constant folding is either a constant or a lexically-scoped scalar which has no other references, then it will be used in place of function calls made without & or do. Calls made using & or do are never inlined. (See constant.pm for an easy way to declare most constants.)

If you redefine a subroutine which was eligible for inlining you'll get a mandatory warning. (You can use this warning to tell whether or not a particular subroutine is considered constant.) The warning is considered severe enough not to be optional because previously compiled invocations of the function will still be using the old value of the function. If you need to be able to redefine the subroutine you need to ensure that it isn't inlined, either by dropping the () prototype (which changes the calling semantics, so beware) or by thwarting the inlining mechanism in some other way, such as

sub not_inlined () {
23 if $];
}

Overriding Builtin Functions

Many builtin functions may be overridden, though this should be tried only occasionally and for good reason. Typically this might be done by a package attempting to emulate missing builtin functionality on a non-Unix system.

Overriding may be done only by importing the name from a module--ordinary predeclaration isn't good enough. However, the subs pragma (compiler directive) lets you, in effect, predeclare subs via the import syntax, and these names may then override the builtin ones:

To unambiguously refer to the builtin form, one may precede the builtin name with the special package qualifier CORE::. For example, saying CORE::open() will always refer to the builtin open(), even if the current package has imported some other subroutine called &open() from elsewhere.

Library modules should not in general export builtin names like "open" or "chdir" as part of their default @EXPORT list, because these may sneak into someone else's namespace and change the semantics unexpectedly. Instead, if the module adds the name to the @EXPORT_OK list, then it's possible for a user to import the name explicitly, but not implicitly. That is, they could say

use Module 'open';

and it would import the open override, but if they said

use Module;

they would get the default imports without the overrides.

Note that such overriding is restricted to the package that requests the import. Some means of "globally" overriding builtins may become available in future.

Autoloading

If you call a subroutine that is undefined, you would ordinarily get an immediate fatal error complaining that the subroutine doesn't exist. (Likewise for subroutines being used as methods, when the method doesn't exist in any of the base classes of the class package.) If, however, there is an AUTOLOAD subroutine defined in the package or packages that were searched for the original subroutine, then that AUTOLOAD subroutine is called with the arguments that would have been passed to the original subroutine. The fully qualified name of the original subroutine magically appears in the $AUTOLOAD variable in the same package as the AUTOLOAD routine. The name is not passed as an ordinary argument because, er, well, just because, that's why...

Most AUTOLOAD routines will load in a definition for the subroutine in question using eval, and then execute that subroutine using a special form of "goto" that erases the stack frame of the AUTOLOAD routine without a trace. (See the standard AutoLoader module, for example.) But an AUTOLOAD routine can also just emulate the routine and never define it. For example, let's pretend that a function that wasn't defined should just call system() with those arguments. All you'd do is this:

In fact, if you predeclare the functions you want to call that way, you don't even need the parentheses:

use subs qw(date who ls);
date;
who "am", "i";
ls -l;

A more complete example of this is the standard Shell module, which can treat undefined subroutine calls as calls to Unix programs.

Mechanisms are available for modules writers to help split the modules up into autoloadable files. See the standard AutoLoader module described in AutoLoader and in AutoSplit, the standard SelfLoader modules in SelfLoader, and the document on adding C functions to perl code in perlxs.

SEE ALSO

See perlref for more on references. See perlxs if you'd like to learn about calling C subroutines from perl. See perlmod to learn about bundling up your functions in separate files.